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CSSE 432 -- Computer Networks -- Spring 2005
Midterm Exam Answers
1. [15 points] Internet Protocols:
a) What is the difference between a stop-and-wait protocol and a sliding window protocol?
A stop-and-wait protocol waits for each sent packet to be acknolwedged before sending the
next packet. In a sliding window protocol a limited number of messages are sent before
receiving any acknowledgements.
b) Which layers in the Internet protocol stack are processed by each of the following:
i) Router 1-3
ii) End system 1-5
iii) Server 1-5
c) Name the transport protocol used by each of the following applications:
i) Email -- TCP
i) WWW -- TCP
ii) DNS -- UDP
ii) Internet Telephony -- UDP
iii) FTP -- TCP
2. [10 points] TCP Congestion Control
a) Name all the types of events that cause the TCP congestion window to increase in value.
Indicate how much does it increases for each type of event.
New ACK -- if the congestion window is currently below the threshold, then
increase by 1 MSS for each successful ACK (exponential growth), if congestion
window is at or above the threshold, then increase linearly (1 MSS for each RTT)
b) Name all the types of events that cause the TCP congestion window to decrease in value.
Indicate how much it decreases for each type of event.
Triple duplicate ACKs -- half of its current value
Timeout -- set to 1 MSS
3. [10 points] Checksums
0000011100001011
0000110000000101
0000100000001010
0000111100000101
0000101100001001
0000101000000011
----------------------------------0 0 1 1 1 1 1 1 0 0 1 0 1 0 1 1 sum
1 1 0 0 0 0 0 0 1 1 0 1 0 1 0 0 complement
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CSSE 432 -- Computer Networks -- Spring 2005
Midterm Exam Answers
4. [25 points] Distance Vector Protocol
Given the following network, compute the distance tables for the first 3 stages of the distance
vector algorithm.
X
2
7
Z
3
Y
∞ ∞
∞ ∞
X
X 0
Y 7
Z 2
to
Y Z
5 2
0 3
3 0
X
X 0
Y 5
Z 2
to
Y Z
7 2
0 3
3 0
X
X 0
Y 7
Z 2
to
Y Z
7 2
0 3
3 0
from
X
X 0
Y ∞
Z ∞
to
Y Z
7 2
from
from
Node X
X
X 0
Y 5
Z 2
to
Y Z
5 2
0 3
3 0
X
X 0
Y 5
Z 2
to
Y Z
5 2
0 3
3 0
X
X 0
Y 5
Z 2
to
Y Z
5 2
0 3
3 0
X ∞
Y 7
Z ∞
Z
∞ ∞
0
3
from
from
X
to
Y
∞ ∞
from
Node Y
X ∞
Y ∞
Z 2
Z
∞ ∞
∞ ∞
3
0
from
from
X
to
Y
2
from
Node Z
CSSE 432 -- Computer Networks -- Spring 2005
Midterm Exam Answers
5. [40 points] Vocabulary
11
iterated query
2
circuit switching
8
fragmentation or
segmentation
multiplicative
decrease
Domain Name
System (DNS)
flow control
4
cookie
19
23
6
service model
stop-and-wait
Dynamic Host
Configuration
Protocol (DHCP)
virtual circuit
query flooding
12
5
25
17
bootstrap node
15
3
Post Office
Protocol 3
(POP3)
Internet Mail
Access Protocol
(IMAP)
Simple Mail
Transport
Protocol (SMTP)
HyperText
Transfer Protocol
(HTTP)
Selective Repeat
(SR)
pipelining
conditional get
7
fast retransmit
16
propagation
delay
queuing delay
socket
10
20
9
18
22
a method of handling unresolved queries for DNS information where
the contacted server does not try to resolve the name but passes back
the name of another server to ask
a type of network where the resources along a path are reserved for
the duration of the communication session
breaking large packets into smaller segments in order to pass them
onto another network
decreasing transmission rate quickly (halving it) when congestion is
observed
distributed database of mapping between host names and IP numbers
a mechanism to prevent either side of a connection from overwhelming
the other
identifier that is associated with a user so that web server can keep
track of state of session or previous history of user
the guarantees that a protocol layer provides
waiting for an acknowledgment before sending the next packet
get IP address dynamically from server
source-to-destination path behaves much like telephone circuit
method of propagating queries to other peers in a distributed P2P
network
node in a P2P network that provides connection to other nodes when
a new peer joins the network
protocol used between mail server and a user agent where the server
keeps no state about the user between sessions
protocol used between mail server and a user agent where the server
keeps track of the user's state between sessions
protocol used between mail servers to exchange email
protocol used by web browsers and servers
protocols that only retransmit packets that were lost or corrupted
sending multiple packets without waiting for acknowledgments
server checks modification date of requested object; if client has latest
version then no object is sent
TCP retransmits a missing segment before its timer expires when 3
duplicate ACKs are received
the time required to move a bit along a link
the time that a packet spends in a queue waiting to be transmitted
a host-local; application-created; OS-controlled interface into which an
application process can send and receive messages from another
application process
3
CSSE 432 -- Computer Networks -- Spring 2005
Midterm Exam Answers
Here are the terms that were not matched:
1 additive increase
13 Network Address
Translation (NAT)
14 out-of-band
21 slow start
24 User Datagram Protocol
(UDP)
increasing the transmission rate slowly (adding a small amount)
when congestion is not observed
mapping several host IP addresses to one IP address and new
port numbers so that the local network can be hidden
a method of signaling that uses a separate channel than the
channel containing the data
beginning of TCP transmission at a slow rate
unreliable data transfer protocol that provides faster response
than TCP
4